[prestadude]

It is likely a vacuum pump. I had my rig in to Newell for service with two complaints,1) there was something constantly running (sounds like a compressor) underneath my feet in the drivers position and 2) air flow from the dash vents was poor. Turns out the vacuum pump (which sounds just like a 12V compressor) was bad and could not generate a vacuum and consequently ran all the time. The vacuum is used to move the blend doors in the dash vent system. Replacing the vacuum pump fixed both problems.